The amount of nitrogen in an organic substance can be determined by an analytica

Question

The amount of nitrogen in an organic substance can be determined by an analytical method called the Kjeldahl method, in which all the nitrogen in the organic substance is converted to ammonia. The ammonia, which is a weak base, can be neutralized with hydrochloric acid, as described by the equation; NH_3(aq)+HCl(aq) rightarrow NH_4 Cl(aq) If 54.0 mL of 0.150 M HCl(aq) is needed to neutralize all the NH_3(g) from a 2.25-s sample of organic material, calculate the mass percentage of nitrogen in the sample.

Explanation / Answer

organic substance (nitrogen only) --> NH3

NH3 + neutraization with HCl

strategy--> amount of acid used --> amount of NH3 present --> relate to Nitrogen

mol of HCl = MV = 54*0.15 = 8.1 mmol of HCl

then we have 8.1 mmol of NH3

1 mol of N is present in 1 mol of NH# so

8.1 mmol of N are present

mass of N = mol*MW = (8.1*10^-3)(14) = 0.1134 g of N

if mass asmple = 2.25

then

% N = mass of N /total mass * 100 = 0.1134/2.25 *100 = 5.04 % of N

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